Rendering outdoor world in Gazebo

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How would you simulate an outdoor environment with the Gazebo simulator? e.g. I would need to provide a texture map of the ground, sky and background, so that the robot sees more than just a flat color shade. The tutorials explain how to populate a simple world with objects, but they all seem to assume I'm simulating an indoor environment with no background or special lighting conditions. I can't find how to texture the ground/sky/background to mimic an outdoor environment.

I'm thinking about participating in an autonomous vehicle competition, and I'm trying to find a way to simulate the course so I can easily test how my "virtual" robot reacts to camera input that roughly mimics input from the real course.